The use of transistors in the area of highly integrated or multifunctional circuits requires a reduction in the dimensions of the components and an increase in the speed of the function of the components. Besides the usual bipolar transistors (BJT), interest is increasingly paid to the heterobipolar transistors (HBT) as well.
A bipolar transistor can be operated up to a maximum frequency of fmax, at which its power amplification has dropped to 1.
The frequency fmax is inversely proportional to the square root of the product of the capacitance C.sub.BC between the base B and the collector C and the base resistance R.sub.B. The collector-base capacitance is determined by the size of the emitter surface and the parasitic capacitances, which are inevitably caused by the base contacts. The resistance R.sub.B is determined essentially by the doping and the thickness of the base, the conductivity of the metallic lead and the contact resistances.
To reduce parasitic collector capacities, it has been known from EP 0 355 799 A2 and U.S. Pat. No. 4,663,831 that the active transistor areas with equal lateral dimensions can be increased by etching a vertical groove in a series of layers of polycrystalline semiconductor material and/or insulating material and depositing the series of transistor layers in the groove. The leads to deeper transistor layers are formed by separate horizontal zones.
It has been known from EP 0 189 136 that a highly doped polycrystalline zone surrounding the base area is provided as a diffusion source for preparing highly doped base contact areas, and the base lead is formed by these base contact areas, the polycrystalline zone and a silicide layer, in order to reduce the base lead resistance of a series of transistor layers.